The power that can be imparted to a body in an electric field is a function of the frequency of the applied power and the square of the electric field E. However, the value of E is practically limited by the risk of dielectric breakdown and resultant corona discharge that can cause ignition of the material being heated. Thus, in a practical sense, if it is desired to raise the amount of power dissipated in a material for purposes of heating it, it is necessary to increase the frequency of the applied power.
Practically speaking, there are two frequency bands that can be used for heating, and these bands involve very different technologies. High frequencies of from 1 to 100 MHz, using conventional dielectric heating techniques, can be utilized. Extremely high frequencies, for example, micro-wave frequencies, can be utilized using shielded transmission lines or wave guides in place of the standard inductances and capacitances in dielectric heating arrangements.
Although it seems more desireable to utilize energy in the micro-wave spectrum, up until the present, there have been restraints which have limited its use.
Previously proposed heating apparatus have utilized cells comprising progressive wave systems (for example, for 2450 MHz, wave guides of the RG 112 type with a section of 43.times.86 mm) or interconnected sections of wave guides that form resonant cavities. These cells cause the formation of a very precise distribution of the electric field within the cell and as a result, the usable zone of the cell is reduced. Also, if the dimensions of the products to be treated are large (for example, for shapes with dense cross sections, with the cross sectional areas exceeding 15 mm.sup.2), the consequent disturbances to the propagation of the electromagnetic waves renders the system useless.
To overcome these shortcomings, it has been proposed to utilize a multimodal cavity comprising a mettalic parallelepipedal box of copper, aluminum or other light alloy, connected to a suitable generator by an appropriate wave guide element. The three dimensions of the enclosure are multiples of 1/4 the wavelength of the energy supplied by the generator. Such apparatus are of larger dimension and allow the heating of shapes of larger cross section, but generally have low production rates.